The Rate of the Reaction between Cn and at Interstellar 22 Temperatures

The rate coefficient for the important interstellar reaction between CN and has been calculated 22 as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques ; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at Ðrst increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50È100 K. The agreement between theory and experiment provides strong conÐrmation that the reaction occurs appreciably at cool interstellar temperatures. Subject headings : ISM: abundances È ISM: clouds È ISM: molecules È molecular processes